AUC ratio (mean ± SD) calculated for each method and for both whole-blood and plasma curves for each tracer.

<p>The AUC ratio is on average more accurate for blood-based methods than for blood-free methods. For [¹¹C]PBR28, but not for [¹¹C](<i>R</i>)-rolipram, the parent AUC ratios of the blood-based methods are less accurate than the whole-blood AUC ratios.</p

Image/blood ratio of the individual rate constants obtained with an unconstrained two-tissue compartment model.

<p>Image/blood ratio of the individual rate constants obtained with an unconstrained two-tissue compartment model.</p

The concentrations over time of [¹¹C](<i>R</i>)-rolipram (A and B) and [¹¹C]PBR28 (C and D) in plasma from the arterial input function (solid line) and from the image input function (dashed line) of a representative healthy subject.

<p>The curves are representative of those from a blood-based (Chen; <b>A and C</b>) and a blood-free (Su; <b>B and D</b>). None of the methods precisely estimated the peak in all the subjects but, in general, blood-based methods yielded a better estimate of the tails of the curves.</p

Transaxial slices from a [¹¹C](<i>R</i>)-rolipram brain scan of a healthy volunteer and from a simulated study using a digital phantom.

<p>Upper row: [¹¹C](<i>R</i>)-rolipram images across the thalamus summed over the whole duration of the scan from a phantom (<b>A</b>) and a healthy volunteer (<b>B</b>). The phantom images are realistic and quite similar to those from the real subjects. The external rim of activity surrounding the brain, in both the subject and the phantom, is scalp activity. Middle row: images summed over the first two minutes at the carotid level. The carotids are well visible near the temporal lobes for both the phantom (<b>C</b>) and the healthy volunteer (<b>D</b>). The regions of high activity visible in the lower part of the cerebellum of the subject (<b>D</b>) are the cerebellar venous sinuses (not simulated in the phantom studies). Bottom row: late images (three summed frames taken at about 1 hour after injection) from a phantom (<b>E</b>) and a subject (<b>F</b>). At late times the carotids are not well visible anymore and the spill-over effect from surrounding tissues becomes more important.</p

Image/blood <i>V</i>_T ratios (mean ± SD) and scores calculated for each method.

<p>The scores are calculated by giving 2 points each time the image/arterial <i>V</i>_T ratio comprised between ±5%, 1 point if comprised between ±5–10%, and 0 points if higher than ±10%. The most accurate results for both tracers were obtained using two blood-based methods (Chen and Mourik) and the Logan plot. When <i>V</i>_T ratios were calculated using these two blood-based methods and an unconstrained two-tissue compartment model (2TCM), the overall results were less accurate. Please note that even if the Chen-[¹¹C]PBR28 score is comparable between the two modeling approaches (11/38 vs. 12/38), the 2TCM yields a greater bias and a greater standard deviation of the mean <i>V</i>_T ratio (1.10±0.17 vs. 1.15±0.20).</p

Results of the phantom simulation.

<p>Results of the phantom simulation.</p

The average concentrations of radioactivity in whole blood (solid line) and parent radioligand in plasma (dashed line) over time for [¹¹C](<i>R</i>)-rolipram (n = 12) (A) and [¹¹C]PBR28 (n = 19) (B).

<p>The main figures show the first 20 minutes of the curves and the inserts the remaining part. Although the shape of the whole blood curves was similar for the two tracers, the relative concentration of parent and metabolites differed. The mean ratio of concentration of parent radioligand in plasma to total radioactivity in whole blood (<b>C</b>) showed that [¹¹C](<i>R</i>)-rolipram remained the predominant component of whole blood radioactivity throughout the scan. In contrast, radiometabolites of [¹¹C]PBR28 became the predominant component of whole blood radioactivity after the first few minutes.</p